Issue Summary

When updating a Delegated LDAP directory it does not accept a blank value on Default Group Memberships if any value is already set, it only accepts another string.

This is reproducible on Data Center: (yes)

Steps to Reproduce

1. In a Jira instance add a Delegated LDAP directory with any value in Default Group Memberships.
2. Try to edit the directory leaving the Default Group Memberships blank.

Expected Results

Directory updated with blank value in Default Group Memberships

Actual Results

It reloads the same value.

Workaround

Workaround 1:

1. Create a second directory with similar values inserted, but leaving the Default Group Memberships and marking to Update User attributes on Login
2. Move the newly created directory to a higher rank than the old
3. Deactivate the old directory after the users login again (so correcting the reference in Jira).

Attention, carefully proceed with this workaround, and only deactivate the old directory after all users login, once (if deactivating first) in Admin > User Management the users will not be available until login again and the reference in issues will point to users with no info. 

Workaround 2: 

1. Create a dummy group that does not grant any permission to the user.
2. Instead of removing the group from the directory configuration, replace with this dummy group.